Carbon and the Molecular Diversity of Life

Introduction to Biomolecules

Living organisms are composed of a vast array of organic compounds, primarily built on the element carbon. These compounds are large, complex, and varied, forming the foundation of biological macromolecules. The four main classes of biomolecules are carbohydrates, lipids, proteins, and nucleic acids. Each class plays a unique and essential role in cellular structure and function.

Carbon Atoms and Molecular Diversity

Bonding Properties of Carbon

Carbon atoms have four valence electrons, allowing them to form up to four covalent bonds with other atoms. This property enables carbon to create a diversity of stable molecules with various shapes and sizes, including chains, rings, and branched structures. The versatility of carbon bonding is foundational to the complexity of organic molecules.

Formation of Bonds with Carbon

The number of covalent bonds an atom can form is determined by its valence, which is the number of unpaired electrons in its outer shell. Carbon can form single, double, or triple bonds, and when two carbons are joined by a double bond, the molecule is planar at that region. Carbon commonly bonds with hydrogen, oxygen, and nitrogen, as well as with other carbon atoms, forming the backbone of organic molecules.

Carbon Skeletons

Carbon chains form the skeletons of most organic molecules. These skeletons can vary in length, branching, the position of double bonds, and the presence of rings. This variability contributes to the immense diversity of organic compounds found in living organisms.

• Length: Number of carbons in the chain

• Branching: Chains may be unbranched or branched

• Double Bond Position: Double bonds can vary in location

• Rings: Some skeletons are arranged in rings

Isomers

Isomers are compounds with the same molecular formula but different structures and properties. There are three main types:

• Structural isomers: Differ in the covalent arrangement of atoms

• Cis-trans isomers: Differ in spatial arrangement around double bonds

• Enantiomers: Mirror images of each other, often with only one form biologically active

Functional Groups

Functional groups are chemical groups attached to carbon skeletons that participate in chemical reactions and confer specific properties to molecules. Seven functional groups are especially important in biological chemistry: hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is an organic molecule consisting of adenosine attached to three phosphate groups. ATP stores potential energy that can be released by hydrolysis, providing energy for cellular processes.

Macromolecules: Polymers and Monomers

Polymers and Monomers

Most macromolecules are polymers, long molecules built from repeating units called monomers. The synthesis and breakdown of polymers involve dehydration (condensation) and hydrolysis reactions, respectively. Enzymes catalyze these reactions.

• Dehydration reaction: Joins two monomers, releasing water

• Hydrolysis reaction: Breaks a bond by adding water

Carbohydrates

Monosaccharides

Carbohydrates are sugars and their polymers. The simplest carbohydrates are monosaccharides, which are classified by the number of carbons and the position of the carbonyl group. Glucose (C6H12O6) is the most common monosaccharide. Monosaccharides serve as major nutrients and building blocks for other molecules.

Ring Formation

In aqueous solutions, most five- and six-carbon sugars, such as glucose, form ring structures, which are the most stable forms under physiological conditions.

Disaccharides

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage. Sucrose (table sugar) is a common disaccharide composed of glucose and fructose.

Polysaccharides

Polysaccharides are carbohydrate polymers with storage or structural roles. Their function depends on the identity of the monomers and the positions of glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers

• Glycogen: Storage polysaccharide in animals, mainly in liver and muscle cells

Structural Polysaccharides

Cellulose is a major component of plant cell walls and the most abundant organic compound on Earth. Chitin is found in arthropod exoskeletons and fungal cell walls, similar to cellulose but with a nitrogen-containing group.

Lipids

Properties and Types of Lipids

Lipids are hydrophobic molecules that do not form true polymers. They are mainly composed of hydrocarbons and include fats, phospholipids, and steroids. Lipids are important for energy storage, membrane structure, and signaling.

Fats (Triacylglycerols)

Fats are constructed from glycerol and three fatty acids via dehydration reactions, forming ester linkages. Fats store more than twice as much energy per gram as carbohydrates.

Saturated vs. Unsaturated Fatty Acids

Fatty acids differ in length and the presence or absence of double bonds:

• Saturated fatty acids: No double bonds, solid at room temperature, mostly animal fats

• Unsaturated fatty acids: One or more double bonds, liquid at room temperature, mostly plant and fish fats

• Trans fats: Unsaturated fats with trans double bonds, often produced industrially

Phospholipids

Phospholipids consist of two fatty acids, a phosphate group, and glycerol. They are essential for cell membrane structure, forming bilayers with hydrophilic heads and hydrophobic tails.

Steroids

Steroids are lipids with a carbon skeleton of four fused rings. Cholesterol is a key steroid in animal cell membranes and a precursor for other steroids such as testosterone and estrogen.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide variety of functions, including defense, storage, transport, communication, movement, and structural support. Each protein has a unique three-dimensional structure that determines its function.

Amino Acids and Polypeptides

Amino acids are organic molecules with carboxyl and amino groups, differing in their side chains (R groups). Proteins are built from 20 different amino acids, linked by peptide bonds to form polypeptides. The sequence of amino acids determines the protein's structure and function.

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids

• Secondary structure: Coils and folds (α helix, β pleated sheet) due to hydrogen bonding

• Tertiary structure: Overall 3D shape from interactions among R groups

• Quaternary structure: Association of multiple polypeptides (e.g., hemoglobin)

Protein Structure and Disease

A single amino acid substitution can drastically affect protein function, as seen in sickle-cell disease, where a change in hemoglobin leads to abnormal red blood cell shape and function.

Denaturation

Protein structure can be affected by environmental factors such as temperature, pH, and salt concentration. Denaturation is the loss of native structure, rendering the protein inactive. Sometimes, denaturation is reversible.

Nucleic Acids

DNA and RNA

Nucleic acids store, transmit, and help express hereditary information. DNA and RNA are polymers of nucleotides. DNA encodes genetic information, while RNA is involved in protein synthesis and gene regulation.

Nucleotide Structure

Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups. Nitrogenous bases are classified as pyrimidines (C, T, U) or purines (A, G). DNA contains deoxyribose; RNA contains ribose.

Polynucleotide Formation

Nucleotides are joined by phosphodiester linkages, forming a sugar-phosphate backbone. DNA strands have directionality (5' to 3').

DNA Structure

DNA consists of two antiparallel polynucleotide strands forming a double helix. Complementary base pairing (A-T, G-C) stabilizes the structure. RNA is usually single-stranded, with uracil replacing thymine.

Genomics and Proteomics

Modern biology uses genomics (study of whole genomes) and proteomics (study of protein sets) to analyze and compare genetic and protein information across species. Bioinformatics tools are essential for handling large datasets.

DNA and Proteins as Evolutionary Tape Measures

DNA sequences are inherited and can be used to assess evolutionary relationships. Closely related species have more similar DNA sequences, allowing molecular biology to trace evolutionary kinship.